Motor vehicle disk brake systems utilize, at each wheel, a brake rotor connected to an axle hub of a rotatable axle of the motor vehicle, and an opposing set of selectively movable brake pads connected to a non-rotating brake caliper which carries a set of brake pads. The brake rotor includes opposing brake pad engagement surfaces, or rotor cheeks, wherein when braking is to occur, the braking system causes the caliper to press the brake pads upon respective brake pad engagement surfaces of the rotor cheek. Frictional interaction between the rotating rotor cheeks and non-rotating brake pads causes braking of the motor vehicle to transpire, the rate of braking depending upon the pressure of the brake pads against the rotor cheeks.
In the automotive art, modern hydraulic braking systems typically include an operator or driver interface, such as a brake pedal. As the driver applies force to this pedal, this force is transmitted by means of control arms and other related devices to the master cylinder. The master cylinder accepts mechanical force as input and produces hydraulic pressure, in the form of pressurized brake fluid, as an output. This pressure is conveyed by means of pressurized brake fluid through lines and valves of the motor vehicle to interface with each brake corner, found near each wheel of the motor vehicle.
FIG. 1A schematically depicts a brake corner 10, known in the art, configured for the usage of a sliding caliper (i.e., piston(s) at one side of the caliper). A brake line 12 conveys hydraulic brake fluid into the brake corner 10. This permits the application of force from the master cylinder (not shown) through pressurization of the hydraulic brake fluid, thereby creating a means of hydraulic control of the hydraulically active components of the brake caliper 20. The hydraulic brake fluid passes into a caliper actuator cylinder 22 and makes contact with a caliper actuator piston 24. The inboard side of the brake caliper 20a is hydraulically active in a sliding caliper configuration, whereas the outboard side of the brake caliper 20b is hydraulically inactive. A brake pad 32a, 32b, is respectively affixed at each side of the brake caliper 20, so that when the hydraulic brake fluid in the brake line 12 supplying the brake corner 10 is pressurized, the brake caliper 20 causes the brake pads to squeeze upon the rotor friction surfaces (i.e., rotor cheeks) 30a of the brake rotor 30, thereby inducing braking of the vehicle. The rotor cheeks 30a, are each located on a respective rotor plate 34a, 34b, mutually separated by vanes 36.
FIG. 1B schematically depicts a brake corner 10′, known in the art, configured for the usage of a fixed caliper (i.e., piston(s) at each side of the caliper). In this case, each side of the brake caliper 20′ is hydraulically active and contains a caliper actuator cylinder 22a, 22b which in turn contains a caliper actuator piston 24a, 24b. A brake pad 32a′, 32b′, is respectively affixed at both sides of the brake caliper 20′ so that when the hydraulic brake fluid is pressurized in the master cylinder, the pressure is transmitted via the hydraulic brake fluid to the caliper actuator pistons 24a, 24b, causing the brake caliper 20′ to engage the brake pads to squeeze upon the cheeks 30a′ of the brake rotor 30′, inducing braking of the vehicle. The rotor cheeks 30a′, are each located on a respective rotor plate 34a′, 34b′, mutually separated by vanes 36′.
Historically, engineering of the human interface with a braking system has been a subjective endeavor. With the advent of a Brake Feel Index (BFI) as reported in SAE technical paper 940331 “Objective Characterization of Vehicle Brake Feel” by D. G. Ebert and R. A. Kaatz (1994), a method was developed to correlate objective engineering parameters to these subjective assessments. In the case of BFI, such aspects as pedal application force, pedal travel and pedal preload are compared to desired target values which correlate to a particular type of response desired and the deviation from these target values is reflected in a lower index value. In disk brake systems, one of the primary causes of undesirable brake pedal feel has been brake pad radial taper wear.
Brake pad (or brake lining) radial taper wear develops with brake usage, wherein the taper angle tends to increase with more aggressive, higher energy brake usage conditions. Brake pad radial taper wear is driven by flexure of the caliper housing under hydraulic pressure, causing a radial pressure gradient over the friction surface by differences in sliding speed over the friction surfaces and by distortion of brake corner components under braking and/or thermal loads, including knuckle abutment distortion and brake rotor coning. Sliding caliper applications will tend to develop most of their radial taper wear on the outboard side, and fixed caliper applications will tend to develop more equalized inboard to outboard radial taper wear, wherein the radial taper wear in fixed caliper applications is usually less pronounced than that of the outboard side of sliding caliper applications.
The primary impact that radial taper wear has on the driver is brake torque variation, which can be perceived as brake pulsing, particularly in high energy applications. Other consequences produced on brake feel by radial taper wear include, but are not limited, to excessive pedal travel and excessive pedal force required in high energy brake applications. It is possible to partially mitigate the effects promoting radial taper wear by optimizing the pad shape, i.e., using a fan shaped pad. However, in many applications it is impractical to impossible to fully stop radial taper wear via pad shape.
Also known in the art is the practice of modifying the brake rotor surface mechanically by cutting grooves into the surface of the rotors, or by drilling holes (i.e., cross-drill holes) forming patterns of holes in a particular configuration. These modifications have been used to increase the friction between the frictional surfaces of the rotor and the brake pad to enhance the removal of heat from the frictional surfaces for purpose of prolonging life of the brake pad material, or to facilitate the clearing of debris which may build up over time between the brake frictional surfaces. Another application of placing grooves in the head is to reduce vibration during braking, wherein the grooves are used to provide a means through which the stresses on the brake pad are balanced while not impairing its coefficient of friction.
Accordingly, what remains needed in the art is a means to enhance the surface characteristics of the friction surfaces of disk braking systems to reduce the radial taper wear behavior of the brake pad surfaces, through a balancing or evening out of the brake pad surface wear.